As is well known, hexavalent chromium (Cr+6) is one of the mostly widely used corrosion inhibitors for metal substrates. Hexavalent chromium has long provided outstanding corrosion resistance in combination with low cost.
However, hexavalent chromium is known to be extremely toxic to humans as well as polluting to the environment. Accordingly, there continues to be ongoing research to find corrosion-resistant coatings which are both chromium-free and economical.
For example, U.S. Patent Application No. US 2004/0022950 A1 to Jung et al. discloses coating metal substrates by applying a formulation which can include aminoalkyl-functionalized siloxanes in combination with metal oxide particles. Some of the metal oxide particles disclosed therein include, for example, cerium oxide and lanthanum oxide particles. The initially coated metal substrate is disclosed as being optionally thermally cured.
The coating in Jung et al. is disclosed as having a thickness of 0.01 μm to 10 μm, i.e., 10 nm to 10,000 nm thickness. Jung et al. also disclose a preferred coating thickness of 0.1 to 6 μm, and a most preferred thickness of 0.7 to 2 μm, i.e., 700 to 2,000 nm thickness. Accordingly, Jung et al. disclose covering a metal surface with a film preferably of micron range thickness.
However, in order to enhance the properties of metal surfaces selected for a particular purpose, there is still a need for ultrathin, for example, less than 10 nm thickness, chromium-free coatings which can impart corrosion resistance comparable to chromium. There also remains a need for a convenient and economical method for producing such ultrathin corrosion-resistant films.